Add a default writing user_id to the config

core-writer/src/config.cpp

@@ -22,4 +22,7 @@ namespace config {
     hsize_t dataset_increase_step = 1000;
     // To which value to initialize a dataset size.
     hsize_t initial_dataset_size = 1000;
+
+    // -1 means to stay with the same user_id.
+    int default_writing_user_id = -1;
 }